In practice, an additional antenna for a mobile phone or some other mobile communication device is most often used in a vehicle as the base station's field strength is low within the body of the vehicle. The additional antenna is in that case naturally situated outside the body of the vehicle, attached thereto. For using an external antenna the vehicle may include a fixed holder so that a phone placed in the holder will be connected to the external antenna through a cable. A holder designed mainly for hands-free operation may also include connection means for an external antenna.
For the purpose of connecting an additional antenna a mobile station may include a coaxial connector in conjunction with the antenna port. Such a connector arrangement based on galvanic contact is, however, relatively expensive and unreliable in the long term. Instead of galvanic contact, electromagnetic coupling can be applied. From patent document GB 2,266,997 is known a solution according to FIG. 1. Therein, a coupling part 110 is attached to the covering of a radio device RD with velcro tape or the like for vehicular use, with a coaxial cable 130 running from said coupling part to an additional antenna, i.e. the external antenna of a car in the case illustrated in that particular patent document. Inside the coupling part 110 there is a conductive loop shaped such that there is a notable inductive coupling between the internal antenna ANT of the radio device and the conductive loop. Radio frequency energy is transferred via the inductive coupling to the external antenna during transmitting and from the external antenna into the radio device during receiving. A disadvantage of this solution is that the attachment of the coupling part may significantly shift the operating band of the antenna and degrade the matching of the antenna at least in part of the operating band. Moreover, the strength of the coupling leaves somewhat to be desired. This is emphasized by the fact that transmitting energy in the field of the conductive frame of the radio device is not transferred to the external antenna via the coupling.
From patent document Fl 100927 is known an arrangement according to FIG. 2 for connecting an additional antenna. In the figure there is a radio device RD with an external antenna ANT. The radio device is placed in a holder 250 extended by a coupling element 210 according to the patent in question. The coupling element is formed of a dielectric block the longitudinal opposing surfaces of which are coated with a conductive material. The outer of these coatings is connected to the outer conductor 232 of a coaxial cable belonging to the arrangement, and the other coating is connected to the inner conductor 231 of the coaxial cable. The coupling element 210 partly surrounds the antenna ANT and is positioned at such an angle with respect to the longitudinal axis thereof that the radio frequency field of the antenna ANT is mainly guided to the coupling element via an intermediate coupling hole 215 and the end surface of the coupling element. From the coupling element the field further is guided via said coaxial cable to the additional antenna. The outer surface of the holder 250 is coated with a planar conductive material 220 in galvanic contact with the outer coating of the connecting element 210 and the outer conductor 232 of the cable. The conductive plane 220 has a significant electromagnetic coupling with the conductive frame of the radio device so that transmitting energy in the field of the frame is transferred to the additional antenna.
A drawback of the connection arrangement of FIG. 2 is that it cannot be applied to a radio device having an internal antenna.